Bi-Directional Bridgeless Buck-Boost Converter

ABSTRACT

A bidirectional bridgeless buck-boost power converter circuit is provided that can function as both a voltage source inverter (VSI) circuit to transform direct current (DC) voltage to alternating current (AC) voltage and as a power factor corrector (PFC) circuit to transform AC voltage to DC voltage. The disclosed converter fully utilizes inductors to form a CL filter and buck-boost converter energy storage element. Thus, low inductance chokes are used in the converter, which leads to a higher power density and is more cost-effective. Further, the bridgeless configuration minimizes conduction losses of semiconductors, and coupled with the use of low inductance chokes this improves system efficiency.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to U.S. application No.62/413,262 filed Oct. 26, 2016, the entirety of which is incorporated byreference for all purposes.

TECHNICAL FIELD

The present disclosure relates to bi-directional bridgeless buck-boostconverters for conversion from direct current (DC) voltage tosingle-phase current alternating (AC) voltage and vice versa.

BACKGROUND

Various industrial applications require the conversion of electric powerfrom direct current (DC) voltage to alternating current (AC) voltage andvice versa. This can be achieved using Voltage Source Inverter (VSI)circuits and Power Factor Correction (PFC) circuits, respectively.Modern power electronics converters require high performance, such ashigh efficiency and high power density, and low cost. For someapplications, the requirements are even higher in order to satisfyindustrial standards, such as is the case with leakage current limits.However, most power converter circuits that are designed to try tominimize leakage current consequently reduce efficiency and powerdensity.

In PFC circuits, the power factor of the AC flowing through the circuitshould be corrected to be as close to 1 such that the real powerdelivered to the circuit is the same as the apparent power before beingconverted to DC. Correction of the power factor is typically achievedusing a passive network of capacitors or inductors. The AC voltage isconverted to the DC voltage using some sort of rectifier, however the DCoutput that is generated typically comprises pulses of current becausethe AC is sinusoidal, which accordingly may be smoothed out using afilter (usually some sort of capacitor arrangement).

PFC circuits are used in a wide range of applications, including but notlimited to motor drives, electric vehicle chargers, electronic ballasts,uninterrupted power supply systems, etc. However, several problems existwhen these circuits are implemented. A first problem is a low efficiencyproblem due to high losses in the conversion of the voltage. This isbecause in a power supply unit there are usually two power stages, thefirst being a PFC stage to shape the input current to be sinusoidal tomeet industrial standards and to step up the grid voltage (e.g. 120V) tobe higher than the peak voltage of the grid (e.g. 200V), and the secondstage comprising a buck-converter to step down the voltage (e.g. 48V). Asecond problem is a common mode voltage issue, which is a common problemfor bridgeless converters (used so that the output has the same polarityas the input). Conventional bridgeless converters create high voltagejumping between a negative port of the DC link and system ground, whichleads to high leakage current that fails to meet industrial standards.Attempts have been made to try to correct these issues, but oftenrequire a large number of semiconductors in the current path leading tohigh conduction losses and costs, and/or require large input filters.

Broadly, VSI circuits perform the opposite of PFC circuits in that theyare converting a DC voltage to an AC voltage. Since the voltage is beingconverted from DC, the power factor should already be equal to 1 andthus there is no power factor correction required in AC. VSI circuitsare used in a wide range of applications, including but not limited to:photovoltaic (PV) inverters where the variable DC output of a solar PVpanel is converted into AC to be fed into a commercial electric grid orfor use by an off-grid network, fuel cell inverters which performsimilar function but with fuel cells, other grid-connected applications,etc.

However, several problems exist when these circuits are implemented.Similar to PFC circuits, there are low efficiency problems and commonmode voltage issues. With regards to a PV inverter for example, thereare typically two power stages—the first being a DC-DC boost converterfor Maximum Power Point Tracking (MPPT) to determine the maximum poweroutput from the PV power and to maintain the DC voltage link at a highenough value for the second power stage, which is the VSI comprising abuck-type DC-AC inverter and which shapes the output current as a sinewave to meet industrial standards. The first stage requires a boostconverter to bring up the voltage from the PV panels to the DC linkcapacitor bank. This approach gives an unimpressive efficiency for thewhole converter of maximum 97% for a 1.5 kW system, due to the two powerconversion stages.

Again using a PV inverter as an example, the common mode voltage issueis typical for transformerless PV inverters because there is highvoltage jumping between a negative port of PV panels and the systemground, this leads to a shorter lifetime of the PV panels and createselectromagnetic interference noise to the grids, as well as potentiallyposes a safety issue for users. Thus there are regulations to limit thevalue of the leakage current of a VSI (for example, in Europe: DIN V VDEV 0126-1-1, which limits the leakage current to 300 mA).

Conventionally, using bi-polar switching was a solution to the leakagecurrent issue, however this creates increased switching losses andincreases the size of grid inductors in order to achieve small currentripples. Other attempts have been made to solve these problems butproduce negative effects such as increased conduction losses due to alarge number of semiconductors.

Accordingly, an improved bridgeless buck-boost electrical powerconverter circuit remains highly desirable.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present disclosure will becomeapparent from the following detailed description, taken in combinationwith the appended drawings, in which:

FIG. 1 shows a grid-connected voltage source inverter (VSI) circuitcomprising a bridgeless buck-boost that can transform direct current(DC) voltage to single-phase alternating current (AC) voltage;

FIG. 2 shows a bridgeless buck-boost power factor correction (PFC)circuit that can transform single-phase alternating current (AC) voltageto direct current (DC) voltage;

FIG. 3 provides possible configurations for bi-directional switcharrangements;

FIGS. 4A and 4B show two switching states in the VSI during a positivehalf line cycle;

FIGS. 5A and 5B are corresponding equivalent circuits of FIGS. 4A and4B, respectively;

FIGS. 6A and 6B show two switching states in the VSI during a negativehalf line cycle;

FIGS. 7A and 7B are corresponding equivalent circuits of FIGS. 6A and6B, respectively;

FIGS. 8A and 8B show two switching states in the PFC during a positivehalf line cycle;

FIGS. 9A and 9B are corresponding equivalent circuits of FIGS. 8A and 8Brespectively;

FIGS. 10A and 10B show two switching states in the PFC during a negativehalf line cycle;

FIGS. 11A and 11B are corresponding equivalent circuits of FIGS. 10A and10B respectively;

FIGS. 12A thru 12E show variants of the VSI circuit shown in FIG. 1;

FIGS. 13A thru 13E show variants of the PFC circuit shown in FIG. 2;

FIGS. 14A thru 14D show the simulation results of the PV invertercircuit;.

FIGS. 15A and 15B show exemplary isolation circuits for the powerconverter circuits described herein;

FIG. 16 shows the simulation results for the VSI circuit of FIG. 1 inbuck mode;

FIG. 17 shows the simulation results for the VSI circuit of FIG. 1 inbuck-boost mode;

FIG. 18 shows the simulation results for the PFC of FIG. 2 in boostmode; and

FIG. 19 shows the simulation results for the PFC of FIG. 2 in buck-boostmode.

It will be noted that throughout the appended drawings, like featuresare identified by like reference numerals.

DETAILED DESCRIPTION

The present disclosure provides an electric power conversion circuit,comprising: a direct current (DC) voltage connection interface providinga positive DC port and a negative DC port; an alternating current (AC)voltage connection interface providing a positive AC port and a negativeAC port; a first series connection having a first diode (D1) with afirst inductor (L1), connected in parallel with the positive DC port andnegative DC port of the DC voltage connection; a second seriesconnection having a second diode (D2) with a second inductor (L2),connected in parallel with the positive DC port and negative DC port ofthe DC voltage connection; a first switch (S3) having a first endconnected to a point along the first series connection between the firstdiode (D1) and the first inductor (L1), and having a second endconnected to the positive AC port of the AC voltage connection; a secondswitch (S4) having a first end connected to a point along the secondseries connection between the second diode (D2) and the second inductor(L2); and a reconfigurable capacitor-inductor (CL) filter circuitconfigured to connect one or more switching capacitors (CAB; CA, CB)between the AC ports and the DC ports.

The invention will now be described in detail with reference to variousembodiments thereof as illustrated in the accompanying drawings.Specific details are set forth in order to provide a thoroughunderstanding of the invention. It will be apparent to one skilled inthe art that the invention may be practiced without using some of theimplementation details set forth herein. It should also be understoodthat well known operations have not been described in detail in order tonot unnecessarily obscure the invention. Embodiments are describedbelow, by way of example only, with reference to FIGS. 1-19.

The present disclosure provides a bidirectional bridgeless buck-boostpower converter circuit, wherein bidirectional means that the convertercircuit can function as both a voltage source inverter (VSI) circuit totransform DC voltage to AC voltage and as a power factor corrector (PFC)circuit to transform AC voltage to DC voltage. The disclosed converterfully utilizes inductors to form a CL filter and buck-boost converterenergy storage element. Thus, low inductance chokes are used in theconverter, which leads to a higher power density and is morecost-effective. Further, the bridgeless configuration minimizesconduction losses of semiconductors, and coupled with the use of lowinductance chokes this improves system efficiency.

FIG. 1 shows a grid-connected voltage source inverter (VSI) circuitcomprising a bridgeless buck-boost that can transform direct current(DC) voltage to single-phase alternating current (AC) voltage. Thiscould be used in several applications including but not limited to PVinverters, fuel cell inverters, etc. The VSI circuit receives inputvoltage from a DC source having positive and negative ports V_(DC+) andV_(DC−), respectively, at a DC voltage connection interface. The outputvoltage is AC, with output ports corresponding to the positive andnegative AC ports at an AC voltage connection interface. The VSIconverter circuit comprises four high frequency semi-conductor switches(S1-S4), two low frequency bi-directional switching semiconductorswitches (SA and SB), two inductors (L1 and L2), and one capacitor(CAB). The DC connection interface may comprise one or more capacitors(C) between the positive and negative ports. The VSI circuit may bebroken down into two main sub-components, a bridgeless buck-boostcircuit 102, and a reconfigurable capacitor-inductor (CL) filter circuit104 which may also be referred to as an active virtual ground (AVG)circuit.

The bridgeless buck-boost circuit 102 comprises a capacitor, C, at theDC voltage connection interface between the positive (V_(DC+)) andnegative (V_(DC−)) ports. There are two series connections comprisingswitches S1 and S2 respectively in series with two inductors L1 and L2.The switches comprise a high-frequency semiconductor switch withparasitic anti-parallel diodes. The polarity of these switches are suchthat the switches can block current from flowing through the switches tothe negative DC voltage input port V_(DC−). The inductors can be used asa buck-boost energy storage inductor or a grid filtering inductordepending on the polarities. These series connections are in parallelwith the positive and negative ports of the DC voltage source V_(in).The bridgeless buck-boost circuit 102 further comprises two additionalswitches S3 and S4 (again each comprising a high-frequency semiconductorswitch with a parasitic anti-parallel diode) which have a first endconnected along the series connections of the switches S1, S2 and theinductors L1, L2. In particular, switch S3 has a first end connected ata point between the switch S1 and inductor L1, and switch S4 has a firstend connected at a point between the switch S2 and inductor L2. A secondend of the switches S3 and S4 connect to AC output ports. In particular,switch S3 has a second end connected to the positive AC output port, andthe switch S4 has a second end connected to the negative AC output port.The switches S3 and S4 have polarity such that they can block currentfrom flowing to the respective AC output port. That is, switch S3 canprevent current from flowing to the positive AC output port, and switchS4 can prevent current from flowing to the negative AC output port.

The VSI circuit further includes a reconfigurable CL filter circuit 104as a filtering technique. This circuit is configured to connect acapacitor C_(AB) between the DC input ports and the AC output ports.This allows positive current to flow to the positive AC voltage outputport corresponding to a positive part of the AC sine wave, and fornegative current to flow to the negative AC voltage output portcorresponding to a negative part of the AC sine wave. This switching isachieved through the use of low frequency bi-directional switchingsemiconductor switches SA and SB, of which SA is connected in seriesbetween the input DC voltage ports and the positive AC output port, andSB is connected in series between the input DC voltage ports and thenegative AC output port. The bi-directional switches SA and SB may berealized by connecting two MOSFETs back-to-back in series or othercircuits and devices which can provide bi-directional blockingcharacteristics, as will be described with reference to FIG. 3. Thus,the reconfigurable CL filter circuit 104 can formulate two different CLfilter structures depending on grid voltage polarity. This providesgrid-connected capability and reduces the number of components requiredto be used.

The configuration of the VSI circuit provides several benefits whichwill be further discussed herein, including but not limited to lowsemiconductor losses, low leakage current, as well a wide range of inputvoltage control capacity. In particular, the single stage buck-boostcircuit 102 increases efficiency due to the elimination of unnecessarypower stages. The reconfigurable CL filter circuit 104 provides acapacitor in between the DC voltage input and the AC voltage output,which eliminates the conventional common mode voltage issue since thecapacitor in the CL filter clamps the voltage between the grid and thepositive port of the DC voltage connection interface.

FIG. 2 shows a bridgeless buck-boost power factor correction (PFC)circuit that can transform single-phase alternating current (AC) voltageto direct current (DC) voltage. This could be used in severalapplications including but not limited to battery chargers, server powersupplies, AC and DC grid interfaces, etc. The PFC circuit receives inputvoltage from an AC source having positive and negative ports at an ACvoltage connection interface. The output voltage is DC, with outputports corresponding to the positive and negative DC ports V_(DC+) andV_(DC−), respectively at a DC voltage connection interface. The PFCconverter bucks or boosts the AC grid voltage to the DC voltageconnection interface and controls the flow of DC power. The PFCconverter comprises two diodes (D1 and D2), two high-frequencysemiconductor switches (S3 and S4), two low frequency bi-directionalswitching semiconductor switches (SA and SB), two inductors (L1 and L2),and one capacitor (CAB). The DC connection interface may comprise one ormore capacitors (C) between the positive and negative ports. The PFCcircuit may be broken down into two main sub-components, a bridgelessbuck-boost circuit 202, and a reconfigurable CL filter circuit 204.

The bridgeless buck-boost circuit 202 comprises a capacitor, C, at DCvoltage connection interface between the positive (V_(DC+)) and negative(V_(DC−)) ports. There are two series connections comprising diodes D1and D2 respectively in series with two inductors L1 and L2. The polarityof these diodes are such that the diodes can block current from flowingto the negative DC voltage output port V_(DC−). The inductors can beused as a buck-boost energy storage inductor or a grid filteringinductor depending on the polarities. These series connections are inparallel with DC voltage connection interface. The bridgeless buck-boostcircuit 102 further comprises two switches S3 and S4 comprising ahigh-frequency semiconductor switch with a parasitic anti-paralleldiode, which have a first end connected along the series connections ofthe diodes D1, D2 and the inductors L1, L2. In particular, switch S3 hasa first end connected at a point between the diode D1 and inductor L1,and switch S4 has a first end connected at a point between the diode D2and inductor L2. A second end of the switches S3 and S4 connect to ACinput ports. In particular, switch S3 has a second end connected to thepositive AC input port, and the switch S4 has a second end connected tothe negative AC input port. The switches S3 and S4 have polarity suchthat they can block current from flowing from the respective AC inputport. That is, switch S3 can prevent current from flowing from thepositive AC input port, and switch S4 can prevent current from flowingfrom the negative AC input port.

The PFC circuit further includes a reconfigurable CL filter circuit 204as a filtering technique. This circuit is configured to connect acapacitor C_(AB) between the AC input ports and the DC output ports.This allows positive current to flow from the positive AC voltage inputport corresponding to a positive part of the AC sine wave, and fornegative current to flow from the negative AC voltage input portcorresponding to a negative part of the AC sine wave. This switching isachieved through the use of low frequency bi-directional switchingsemiconductor switches SA and SB, of which SA is connected in seriesbetween the positive input AC voltage port and the DC output ports, andSB is connected in series between the negative input AC voltage port andthe DC output ports. The bi-directional switches SA and SB may berealized by connecting two MOSFETs back-to-back in series or othercircuits and devices which can provide bi-directional blockingcharacteristics, as will be described with reference to FIG. 3. Thus,the reconfigurable CL filter circuit 204 can formulate two different CLfilter structures depending on grid voltage polarity. This providesgrid-connected capability and reduces the number of components requiredto be used.

The configuration of the PFC circuit provides several benefits whichwill be further discussed herein, including but not limited to lowsemiconductor losses, low leakage current, as well a wide range ofoutput voltage control capacity. In particular, the single stagebuck-boost circuit 202 increases efficiency due to the elimination ofunnecessary power stages. The reconfigurable CL filter circuit 204provides a capacitor in between the AC voltage input and the DC voltageoutput, which eliminates the conventional common mode voltage issuesince the capacitor in the CL filter clamps the voltage between the gridand the positive port of the DC voltage connection interface.

FIG. 3 provides possible configurations for bi-directional switch (SA orSB) arrangements. Any of these configurations could be implemented toobtain the functionality of aforementioned bi-directional switches SAand SB. Configuration 302 represents an ideal switch, configurations 304and 306 are alternatives that could be used in the PFC circuit or in theVSI circuit if PF=1 (for reactive power control they do not make sense),configuration 308 represents a diode bridge, configuration 310represents a common emitter back-to-back, configuration 312 represents acommon drain back-to-back, and configuration 314 representsanti-paralleled reverse blocking insulated-gate bipolar transistors(IGBTs).

Switching states and equivalents are now described to exemplify some ofthe functionality and operation of the VSI and PFC circuits shown inFIGS. 1 and 2.

FIGS. 4A and 4B show two switching states in the VSI during a positivehalf line cycle. In this half line cycle, the filter capacitor (CAB) isconnected to line (+) of the grid voltage through the bi-directionalswitch (SA), the switch (S4) is always conducting and the switch (S2) isalways open. The switch (S1) is switching at a high frequency and theanti-paralleled diode of S3 is conducting alternatively. FIGS. 4A and 4Bshow the circuit when the switch (S1) is switched on and off,respectively.

FIGS. 5A and 5B are corresponding equivalent circuits of FIGS. 4A and4B, respectively. It can be seen that CAB and L2 form a CL filterbetween the grid and the buck-boost converter. L1 is a buck-boostconverter storage element in order to absorb the energy from the DC linkand release it to the filtering capacitor CAB. Capacitor CAB is coupledbetween line (+) and the positive port of DC voltage connectioninterface, the potential difference between them is clamped, thereforelow leakage current is achieved.

FIGS. 6A and 6B show two switching states in the VSI during a negativehalf line cycle. In this half line cycle, the filter capacitor (CAB) isconnected to Neutral (N) of the grid voltage through the bi-directionalswitch (SB), the switch (S3) is always conducting and the switch (S1) isalways open. The switch (S2) is switching at a high frequency and theanti-paralleled diode of S4 is conducting alternatively. FIGS. 6A and 6Bshow the circuit when the switch (S2) is switched on and off,respectively.

FIGS. 7A and 7B are corresponding equivalent circuits of FIGS. 6A and6B, respectively. It can be seen that CAB and L1 form a CL filterbetween the grid and the buck-boost converter. L2 is a buck-boostconverter storage element in order to absorb the energy from the DC linkand release it to the filtering capacitor (CAB). Capacitor CAB iscoupled between line (+) and the positive port of DC voltage connectioninterface, the potential difference between them is clamped, thereforelow leakage current is achieved.

Table 1 below summarizes the switching actions of all of the switches inthe four-quatral operating modes for the VSI. The four-quatral operatingmodes allows the VSI to deliver reactive power. In Table 1, “1”represents high, “0” represents low, “X” represents it does not matter,“HF” represents high frequency, and “LF” represents low frequency.

TABLE 1 Switch- Input ing Volt- Input State age Current SA SB S1 S2 S3S4 L1 L2 CDC 1 +ve +ve 1 0 1 0 0 1 HF LF Dis- charge 2 +ve +ve 1 0 0 0 01 HF LF Open 3 +ve −ve 1 0 0 0 1 X HF LF Open 4 +ve −ve 1 0 0 0 0 X HFLF Charge 5 −ve −ve 0 1 0 1 1 0 LF HF Dis- charge 6 −ve −ve 0 1 0 0 1 0LF HF Open 7 −ve +ve 0 1 0 0 X 1 LF HF Open 8 −ve +ve 0 1 0 0 X 0 LF HFCharge

FIGS. 8A and 8B show two switching states in the PFC during a positivehalf line cycle. In this half line cycle, the filter capacitor (CAB) isconnected to Line (+) of the grid voltage through the bi-directionalswitch (SA), the switch (S4) is always conducting and the switch (S3) isswitching at a high frequency. FIGS. 8A and 8B show the circuit when theswitch (S3) switching on and off, respectively.

FIGS. 9A and 9B are corresponding equivalent circuits of FIGS. 8A and 8Brespectively. It can be seen that CAB and L2 form a CL filter betweenthe grid and the buck-boost converter. L1 is a buck-boost converterstorage element in order to absorb the energy from the filtering CAB andrelease it to the DC link. Capacitor CAB is coupled between line (+) andthe positive port of DC voltage connection interface, the potentialdifference between them is clamped, therefore low leakage current isachieved.

FIGS. 10A and 10B show two switching states in the PFC during a negativehalf line cycle. In this half line cycle, the filter capacitor (CAB) isconnected to Neutral (N) of the grid voltage through the bi-directionalswitch (SB), the switch (S3) is always conducting and the switch (S4) isswitching at a high frequency. FIGS. 10A and 10B show the circuit whenthe switch (S4) switching on and off, respectively.

FIGS. 11A and 11B are corresponding equivalent circuits of FIGS. 10A and10B respectively. It can be seen that CAB and L1 form a CL filterbetween the grid and the buck-boost converter. L2 is a buck-boostconverter storage element in order to absorb the energy from thefiltering capacitor (CAB) and release it to the DC link. Capacitor CABis coupled between Neutral (N) and the positive port of DC voltageconnection interface, the potential difference between them is clamped,therefore low leakage current is achieved.

In addition to the VSI and PFC circuits shown in FIGS. 1 and 2 withswitching states as described with respect to FIGS. 4-11, severalvariants exist which provide similar performance. These are shown inFIGS. 12 and 13.

FIGS. 12A thru 12E show variants of the VSI circuit shown in FIG. 1. Inparticular, FIG. 12A shows the bi-directional switches SA and SB on thebottom and each connected in series with a respective capacitor CA andCB instead of a single capacitor CAB. The two capacitors CA and CBconduct alternately. FIG. 12B is the same as FIG. 12A but the switchesSA and SB are again located at the top similar to FIG. 1. FIG. 12C showsthe connection of capacitor CAB to a middle point of the DC voltageconnection interface, and two capacitors C1 and C2 on the DC voltageconnection interface. The connection of capacitor CAB can be at anylocation along the DC voltage connection interface. FIG. 12D shows oneswitch SB and capacitor CB located at the top and the other switch SAand capacitor CA located at the bottom. FIG. 12E shows the connection of12C with one capacitor CB, but other capacitor CA connected to the DCvoltage connection interface as shown in FIG. 1. Numerous otherconfigurations could be readily envisioned by one of ordinary skill inthe art that would provide the same or similar functionality as what isdisclosed herein and would not depart from the scope of this disclosure.

FIGS. 13A thru 13E show variants of the PFC circuit shown in FIG. 2. Inparticular, FIG. 13A shows the bi-directional switches SA and SB on thebottom instead of the top. FIG. 13B shows the switches SA and SB on thebottom and each connected in series with a respective capacitor CA andCB instead of a single capacitor CAB. The two capacitors CA and CBconduct alternately. FIG. 13C shows the switches SA and SB on the bottomand the connection of capacitor CAB to a middle point of the DC voltageconnection interface. FIG. 13D shows one switch SA and capacitor CAlocated on the top and one switch SB and capacitor CB located at thebottom. FIG. 13E shows the same as FIG. 13D with the connection ofcapacitor CB to a point along the DC voltage connection interface.Numerous other configurations could be readily envisioned by one ofordinary skill in the art that would provide the same or similarfunctionality as what is disclosed herein and would not depart from thescope of this disclosure.

FIGS. 14A thru 14D show the simulation results of a PV inverter circuitcomprising the VSI circuit shown in FIG. 1. In particular, FIG. 14Aprovides a power-voltage diagram, which shows that the converter managesto reach a new Maximum Power Point (MPP) rapidly. FIG. 14B provides theDC input voltage, the current, and power waveforms of the VSI circuit.FIG. 14C shows the time domain waveforms for the output voltage andcurrent, as well as the inductor currents. Note that the output currentcan reach a steady state after the operating MPP changed. FIG. 14D showsthat the time domain waveforms (notably the output current waveform) ofFIG. 14C are sinusoidal.

FIGS. 15A and 15B show exemplary isolation circuits for the powerconverter circuits described herein. These circuits perform the samefunctions as described above, however the DC voltage ports and the ACvoltage ports are separated (isolated) by transformers or coupledinductors 1502 and 1504.

FIGS. 16 thru 19 show further simulation results of the bi-directionalbuck-boost converter circuits of FIG. 1 and FIG. 2. In particular, FIG.16 shows the simulation results for the VSI circuit of FIG. 1 in buckmode. FIG. 17 shows the simulation results for the VSI circuit of FIG. 1in buck-boost mode. FIG. 18 shows the simulation results for the PFC ofFIG. 2 in boost mode. FIG. 19 shows the simulation results for the PFCof FIG. 2 in buck-boost mode.

The VSI and PFC circuits disclosed herein, including their variants, maybe implemented as part of any application requiring inverting technologyor power factor correction technology, including but not limited to PVinverters, power supplies, electric vehicle charges, and AC grid and DCgrid interfaces.

It would be appreciated by one of ordinary skill in the art that thecircuits and components shown in FIGS. 1-19 may include components notshown in the drawings. For simplicity and clarity of the illustration,elements in the figures are not necessarily to scale, are only schematicand are non-limiting of the elements structures. It will be apparent topersons skilled in the art that a number of variations and modificationscan be made without departing from the scope of the invention as definedin the claims.

What is claimed is:
 1. An electric power conversion circuit, comprising:a direct current (DC) voltage connection interface providing a positiveDC port and a negative DC port; an alternating current (AC) voltageconnection interface providing a positive AC port and a negative ACport; a first series connection having a first diode (D1) with a firstinductor (L1), connected in parallel with the positive DC port andnegative DC port of the DC voltage connection; a second seriesconnection having a second diode (D2) with a second inductor (L2),connected in parallel with the positive DC port and negative DC port ofthe DC voltage connection; a first switch (S3) having a first endconnected to a point along the first series connection between the firstdiode (D1) and the first inductor (L1), and having a second endconnected to the positive AC port of the AC voltage connection; a secondswitch (S4) having a first end connected to a point along the secondseries connection between the second diode (D2) and the second inductor(L2); and a reconfigurable capacitor-inductor (CL) filter circuitconfigured to connect one or more switching capacitors (CAB; CA, CB)between the AC ports and the DC ports.
 2. The electrical powerconversion circuit of claim 1 wherein the reconfigurable CL filtercircuit comprising two low frequency bi-directional switchingsemiconductor switches (SA, SB) connected in series with the one or morecapacitors, each of the two low frequency bi-directional switchingsemiconductor switches connected to a respective AC port.
 3. Theelectrical power conversion circuit of claim 1, wherein the first andsecond diodes (D1 and D2) are polarized to allows current to flow to thepositive DC port of the DC voltage connection.
 4. The electrical powerconversion circuit of claim 1, wherein the first switch (S3) and secondswitch (S4) each comprise a high-frequency semiconductor switch with aparasitic anti-parallel diode.
 5. The electrical power conversioncircuit of claim 1, wherein when an input voltage is positive the lowfrequency bi-directional switching semiconductor switches (SA and SB)actuate to connect one of the one or more switching capacitors (CAB; CA,CB) to the positive AC port, and wherein when the input voltage isnegative the low frequency bi-directional switching semiconductorswitches (SA and SB) actuate to connect one of the one or more switchingcapacitors (CAB; CA, CB) to the negative AC port.
 6. The electricalpower conversion circuit of claim 5, wherein the two low frequencybi-directional switching semiconductor switches (SA and SB) conductalternately.
 7. The electrical power conversion circuit of claim 1,wherein there are two switching capacitors (CA and CB), each connectedin series with a respective low frequency bi-directional switchingsemiconductor switch (SA and SB).
 8. The electrical power conversioncircuit of claim 1, wherein an input voltage is received at the DCvoltage connection interface and an output voltage is produced at the ACvoltage connection interface.
 9. The electrical power conversion circuitof claim 8, wherein the first and second diodes (D1 and D2) areparasitic anti-parallel components in respective high-frequencysemiconductor switches to form two additional switches (S1 and S2). 10.The electrical power conversion circuit of claim 9, wherein the firstswitch (S3) prevents electricity from flowing to the positive port ofthe AC voltage connection, and wherein the second switch (S4) preventselectricity from flowing to the negative port of the AC voltageconnection.
 11. The electrical power conversion circuit of claim 1,wherein an input voltage is received at the AC voltage connectioninterface and an output voltage is produced at the DC voltage connectioninterface.
 12. The electrical power conversion circuit of claim 11,wherein the first switch (S3) prevents electricity from flowing from thepositive port of the AC voltage connection, and wherein the secondswitch (S4) prevents electricity from flowing from the negative port ofthe AC connection.